Method and system for a pulsed laser source emitting shaped optical waveforms

ABSTRACT

A laser system for processing a workpiece includes a tunable pulsed laser source having an output comprising a set of optical pulses. The tunable pulsed laser source includes a seed source, an optical circulator having a first port coupled to the seed source, a second port, and a third port, a modulator driver, and an amplitude modulator coupled to the modulator driver. The tunable pulsed laser source also includes a first optical amplifier characterized by an input end and a reflective end and a second optical amplifier coupled to the third port of the optical circulator. The laser system also includes a controller configured to adjust laser parameters of the tunable pulsed laser source, a supporting member configured to support the workpiece, and an optical system configured to adjust laser beams from the tunable pulsed laser source and direct them towards the workpiece.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application is a continuation-in-part of and claims the benefit ofco-pending, commonly assigned U.S. patent application Ser. No.11/862,935, filed Sep. 27, 2007, entitled “Method And System For APulsed Laser Source Emitting Shaped Optical Waveforms,” which claims thebenefit under 35 U.S.C. § 119(e) of U.S. Provisional Patent ApplicationNo. 60/848,077, filed Sep. 29, 2006, entitled “Method And System For APulsed Laser Source Emitting Shaped Optical Waveforms.” The entiredisclosures of the above-referenced patent applications are hereinincorporated by reference for all purposes.

BACKGROUND OF THE INVENTION

The present invention relates generally to the field of tunable lasersources. More particularly, the present invention relates to a methodand apparatus for providing high power pulsed laser sources useful forindustrial applications such as trimming, marking, cutting, and welding.Merely by way of example, the invention has been applied to a lasersource with real-time tunable characteristics including pulse width,peak power, repetition rate, and pulse shape. However, the presentinvention has broader applicability and can be applied to other lasersources.

Pulsed laser sources, such as Nd:YAG lasers have been used to performlaser-based material processing for applications such as marking,engraving, micro-machining, and cutting. Depending on the applicationand the materials to be processed, the various characteristics of thelaser pulses, including pulse width, pulse repetition rate, peak poweror energy, and pulse shape, are selected as appropriate to theparticular application. Many existing high power pulsed lasers that arecharacterized by pulse energies greater than 0.5 mJ per pulse, rely ontechniques such as Q-switching and mode locking to generate opticalpulses. However, such lasers produce optical pulses with characteristicsthat are predetermined by the cavity geometry, the mirrorreflectivities, and the like. As such, such laser pulses cannotgenerally be varied in the field without compromising the laserperformance. Using such lasers, it is generally difficult to achieve arange of variable pulse characteristics.

Thus, there is a need in the art for pulsed laser sources with tunablepulse characteristics.

SUMMARY OF THE INVENTION

According to embodiments of the present invention, techniques relatedgenerally to the field of tunable laser sources are provided. Moreparticularly, the present invention relates to a method and apparatusfor providing high power pulsed laser sources useful for industrialapplications such as trimming, marking, cutting, and welding. Merely byway of example, the invention has been applied to a laser source withreal-time tunable characteristics including pulse width, peak power,repetition rate, and pulse shape. However, the present invention hasbroader applicability and can be applied to other laser sources.

According to an embodiment of the present invention, a tunable pulsedlaser source is provided. The tunable pulsed laser source includes aseed source adapted to generate a seed signal and an optical circulatorhaving a first port coupled to the seed source, a second port, and athird port. The tunable pulsed laser source also includes a modulatordriver adapted to produce a shaped electrical waveform and an amplitudemodulator coupled to the modulator driver and adapted to receive theshaped electrical waveform. The amplitude modulator is characterized bya first side coupled to the second port of the optical circulator and asecond side. The tunable pulsed laser source further includes a firstoptical amplifier characterized by an input end and a reflective end.The input end is coupled to the second side of the amplitude modulator.Moreover, the tunable pulsed laser source includes a second opticalamplifier coupled to the third port of the optical circulator.

In a first embodiment, the shaped electrical waveform results in asubstantial reduction in gain saturation impact in the opticalamplifier, thereby providing an optical output pulse characterized by asubstantially square shape. In a second embodiment, the shapedelectrical waveform results in a substantial reduction in gainsaturation impact in the optical amplifier, thereby providing an opticaloutput pulse characterized by an increase in intensity as a function oftime. In a third embodiment, the shaped electrical waveform results in asubstantial reduction in gain saturation impact in the opticalamplifier, thereby providing an optical output pulse characterized by adecrease in intensity as a function of time.

According to another embodiment of the present invention, a method ofproviding a laser pulse is provided. The method includes providing aseed signal, coupling the seed signal to a first port of an opticalcirculator, and outputting the seed signal from a second port of theoptical circulator. The method also includes providing a first shapedelectrical signal, coupling the first shaped electrical signal to anelectrical port of an amplitude modulator, outputting a shaped opticalpulse to an input end of an optical amplifier, and amplifying the shapedoptical pulse to provide an amplified shaped optical pulse. The methodfurther includes providing a second shaped electrical signal, couplingthe second shaped electrical signal to the electrical port of theamplitude modulator, receiving a reshaped optical pulse at the secondport of the optical circulator, and outputting the reshaped opticalpulse at a third port of the optical circulator.

According to a particular embodiment of the present invention, a lasersystem for processing a workpiece is provided. The laser system includesa tunable pulsed laser source having an output comprising a set ofoptical pulses. The tunable pulsed laser source includes a seed sourceadapted to generate a seed signal and an optical circulator having afirst port coupled to the seed source, a second port, and a third port.The tunable pulsed laser source also includes a modulator driver adaptedto produce an electrical waveform and an amplitude modulator coupled tothe modulator driver and adapted to receive the electrical waveform. Theamplitude modulator is characterized by a first side coupled to thesecond port of the optical circulator and a second side. The tunablepulsed laser source further includes a first optical amplifiercharacterized by an input end and a reflective end. The input end iscoupled to the second side of the amplitude modulator. Moreover, thetunable pulsed laser source includes a second optical amplifier coupledto the third port of the optical circulator. The laser system alsoincludes a controller configured to adjust laser parameters of thetunable pulsed laser source and a supporting member configured tosupport the workpiece. The laser system further includes an opticalsystem configured to adjust laser beams from the tunable pulsed lasersource and direct them towards the workpiece.

According to yet another particular embodiment of the present invention,a method of laser processing is provided. The method includes outputtinga first set of optical pulses from a tunable pulsed laser source towarda workpiece. The workpiece, which may also be referred to a substrate,includes at least a first layer of a first material and a second layerof a second material. The first material is different from the secondmaterial. The tunable pulsed laser source includes a seed source adaptedto generate a seed signal and an optical circulator having a first portcoupled to the seed source, a second port, and a third port. The tunablepulsed laser source also includes a modulator driver adapted to producean electrical waveform and an amplitude modulator coupled to themodulator driver and adapted to receive the electrical waveform. Theamplitude modulator is characterized by a first side coupled to thesecond port of the optical circulator and a second side. The tunablepulsed laser source further includes a first optical amplifiercharacterized by an input end and a reflective end. The input end iscoupled to the second side of the amplitude modulator. The tunablepulsed laser source additionally includes a second optical amplifiercoupled to the third port of the optical circulator. The method alsoincludes processing the first layer using the first set of opticalpulses and outputting a second set of optical pulses from the tunablepulsed laser source toward the workpiece. The second set of opticalpulses is different from the first set of optical pulses. The methodfurther includes processing the second layer using the second set ofoptical pulses.

Numerous benefits are achieved using the present invention overconventional techniques. For example, in an embodiment according to thepresent invention, high power, pulsed lasers suitable for laserprocessing are provided that utilize a compact architecture that isinexpensive in comparison to lasers with comparable performancecharacteristics. Moreover, in embodiments of the present invention,short pulses are generated with pulse characteristics that are tunablein real-time while maintaining pulse-to-pulse stability. Furthermore, inan embodiment according to the present invention, optical pulses can beshaped to optimize the pulse profile for the particular application, orto maximize energy extraction efficiency in the laser system. Dependingupon the embodiment, one or more of these benefits may exist. These andother benefits have been described throughout the present specificationand more particularly below. Various additional objects, features andadvantages of the present invention can be more fully appreciated withreference to the detailed description and accompanying drawings thatfollow.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified schematic illustration of a high power pulsedlaser with tunable pulse characteristics using optical fiber amplifiersaccording to an embodiment of the present invention;

FIG. 2 is a simplified timing diagram illustrating electrical andoptical pulses at different locations in a high power pulsed laseraccording to an embodiment of the present invention;

FIG. 3 is a pulse shape diagram for Gaussian and quasi-square(super-Gaussian) pulses;

FIG. 4A is a simplified illustration of an electrical waveform appliedto the amplitude modulator according to an embodiment of the presentinvention;

FIG. 4B is a simplified illustration of an output optical pulse producedin response to the electrical waveform illustrated in FIG. 4A accordingto an embodiment of the present invention;

FIGS. 5A and 5B are simplified illustrations of spiked optical outputpulses according to embodiments of the present invention;

FIGS. 5C and 5D are simplified illustrations of an electrical waveformapplied to the amplitude modulator and the corresponding spiked opticaloutput pulse according to an embodiment of the present invention;

FIG. 6 illustrates sets of optical pulses provided according to variousembodiments of the present invention;

FIGS. 7A and 7B are simplified illustrations of an electrical waveformapplied to the amplitude modulator and the corresponding output opticalwaveforms according to an embodiment of the present invention for threepulses of different energies and widths;

FIGS. 8A and 8B are simplified illustrations of an electrical waveformapplied to the amplitude modulator and an output optical pulse accordingto yet another embodiment of the present invention;

FIG. 9 is a simplified laser system for processing a workpiece accordingto an embodiment of the invention;

FIGS. 10A-10C are simplified illustrations of an exemplary two layerstructure processed by using the simplified laser system shown in FIG.9; and

FIG. 11 is simplified illustration of an exemplary multilayer circuitboard with via holes drilled by using the simplified laser system shownin FIG. 9.

DETAILED DESCRIPTION OF SPECIFIC EMBODIMENTS

FIG. 1 is a simplified schematic illustration of a high power pulsedlaser with tunable pulse characteristics using optical fiber amplifiersaccording to an embodiment of the present invention. High power pulsedlaser 100 includes a seed source 110 that generates a seed signal thatis injected into a first port 114 of an optical circulator 120.According to an embodiment of the present invention, the optical seedsignal is generated by using a seed source 110 that is a continuous wave(CW) semiconductor laser. In a particular embodiment, the CWsemiconductor laser is a fiber Bragg grating (FBG) stabilizedsemiconductor diode laser operating at a wavelength of 1032 nm with anoutput power of 20 mW. In another particular embodiment, the CWsemiconductor laser is an external cavity semiconductor diode laseroperating at a wavelength of 1064 nm with an output power of 100 mW. Inalternative embodiments, the seed signal is generated by a compactsolid-state laser or a fiber laser. One of ordinary skill in the artwould recognize many variations, modifications, and alternatives.

After passing through the optical circulator 120, the seed signal exitsfrom a second port 122 of the circulator 120 and impinges on a firstside 132 of an optical amplitude modulator 130. Circulators are wellknown in the art and are available from several suppliers, for example,model OC-3-1064-PM from OFR, Inc. of Caldwell, N.J.

The optical amplitude modulator 130 is normally held in an “off” state,in which the signal impinging on the modulator is not transmitted.According to embodiments of the present invention, optical amplitudemodulator provides amplitude modulation and time-domain filtering of theseed signal as well as amplified spontaneous emission (ASE) filtering.In a particular embodiment, the length of the optical pulse isdetermined by the operation of the optical amplitude modulator 130,which may be an APE-type Lithium Niobate Mach-Zehnder modulator having abandwidth>3 GHz at 1064 nm.

According to embodiments of the present invention, the optical amplitudemodulator 130 is an electro-optic Mach-Zehnder type modulator, whichprovides the bandwidth necessary for generating short optical pulses. Inother embodiments, the optical amplitude modulator 130 is a phase orfrequency modulator with a suitable phase or frequency to amplitudeconverter, such as an edge optical filter, an extinction modulator, oran acousto-optic modulator.

In order to pass the seed signal, the optical amplitude modulator 130 ispulsed to the “on” state for a first time to generate an optical pulsealong optical path 136. The pulse width and pulse shape of the opticalpulse generated by the optical amplitude modulator 130 are controlledvia by the modulator drive signal applied to the optical amplitudemodulator 130. The optical pulse then passes for a first time through afirst optical amplifier 150, where it is amplified. According toembodiments of the present invention, the amplitude modulator, driven bya time varying drive signal, provides time-domain filtering of the seedsignal, thereby generating a laser pulse with predetermined pulsecharacteristics, including pulse width, pulse shape, and pulserepetition rate.

According to embodiments of the present invention the drive signalapplied to the optical amplitude modulator 130 has a shaped waveformoriginating from a digital pattern converted into an analog signal usinga high speed Digital-to-Analog Converter (DAC). Using a computer, shapedwaveforms are generated by creating a digital representation of thewaveform in the memory on-board the DAC. This digital pattern is thenconverted into an analog signal using a high speed Digital-to-AnalogConverter (DAC). Preferably the DAC's output rise and fall times areless than 1 ns, more preferably less than 500 picoseconds (ps), mostpreferably less than 300 ps. Preferably the DAC is configured togenerate a pre-programmed waveform loaded into memory using a computerevery time a trigger event occurs. Preferably the sampling rate of theDAC is at least 500 megasample/s (MS/s), more preferably it is at least1 gigasample/second (GS/S), most preferably it is at least 2 GS/s. Withsuch a sampling rate, the digital pattern can be defined every 2nanosecond (ns) or better. With 1 GS/s sampling rate, this means thatarbitrary waveform can be generated with 1 ns resolution. Preferably theDAC has an analog electrical bandwidth larger than 100 MHz, morepreferably the analog bandwidth is larger than 300 MHz, and mostpreferably it is larger than 1 GHz. Preferably the voltage resolution ofthe DAC is 8 bits, more preferably it is 10 bits, most preferably it is12 bits or better.

The process flow a user would follow to generate a given electricaldrive waveform, for a particular application is highlighted below. Firsta user would define an array of numbers representing the electricalvoltage applied to the modulator at every 1 nanosecond. It will beapparent to someone skilled in the art that the array length has to beat least as long as the optical pulse width. For example, if the desiredoutput optical pulse is 30 ns, then the electrical waveform will be atleast 30 ns. For a DAC of 1 GS/s sampling rate, the array length willpreferably be more than 30 numbers. Therefore the DAC will generallystore at least 30 samples. Then using a computer, the array of numbersis loaded into the DAC's memory. Then every trigger event would outputthe analog electrical waveform represented by the array of numbers. Theanalog electrical waveform is applied to the modulator. For example, anapparatus for generating such waveform is the model AWG2040 fromTektronix, Inc. of Beaverton, Oreg.

According to an embodiment of the present invention, the opticalamplifier 150 is an optical fiber amplifier. Fiber amplifiers utilizedin embodiments of the present invention include, but are not limited torare-earth-doped single-clad, double-clad, or even multiple-clad opticalfibers. The rare-earth dopants used in such fiber amplifiers includeYtterbium, Erbium, Holmium, Praseodymium, Thulium, or Neodymium. In aparticular embodiment, all of the fiber-optic based components utilizedin constructing optical amplifier 150 utilize polarization-maintainingsingle-mode fiber.

Referring to FIG. 1, in embodiments utilizing fiber amplifiers, a pump142 is coupled to a rare-earth-doped fiber loop 144 through opticalcoupler 140. Generally, a semiconductor pump laser is used as pump 142.One of ordinary skill in the art would recognize many variations,modifications, and alternatives. In alternative embodiments, the opticalamplifier 150 is a solid-state amplifier including, but not limited to,a solid-state rod amplifier, a solid-state disk amplifier or gaseousgain media.

In a particular embodiment, the optical amplifier 150 includes a 5 meterlength of rare-earth doped fiber 144, having a core diameter ofapproximately 4.1 μm, and doped with Ytterbium to a doping density ofapproximately 4×10²⁴ ions/m³. The amplifier 150 also includes a pump142, which is an FBG-stabilized semiconductor laser diode operating at awavelength of 976 nm, and having an output power of 100 mW. In anotherparticular embodiment, the pump 142 is a semiconductor laser diodeoperating at a wavelength of about 915 nm. In yet another particularembodiment, the pump 142 is a semiconductor laser diode operating at anoutput power of 450 mW or more. In a specific embodiment, the amplifier150 also includes a pump to fiber coupler 140, which is a WDM pumpcombiner.

The signal emerging from optical amplifier 150 along optical path 148then impinges on a reflecting structure 146, and is reflected back intooptical amplifier 150. The signal passes for a second time throughoptical amplifier 150, wherein the signal is amplified. The reflectingstructure 146 performs spectral domain filtering of the laser pulse andof the amplified spontaneous emission (ASE) propagating past opticalpath 148. Thus, the seed signal experiences both amplitude andtime-domain modulation passing through amplitude modulator 130, andspectral-domain filtering upon reflection from reflecting structure 146.

In an embodiment, the reflecting structure 146 is a fiber Bragg grating(FBG) that is written directly in the fiber used as the opticalamplifier 150. The periodicity and grating characteristics of the FBGare selected to provide desired reflectance coefficients as is wellknown in the art. Merely by way of example in a particular embodiment,the reflecting structure 146 is a FBG having a peak reflectance greaterthan 90%, and a center wavelength and spectral width closely matched tothe output of the seed source 110.

The signal emerging from optical amplifier 150 along optical path 136impinges on the second side 134 of the optical amplitude modulator 130,which is then pulsed to the “on” state a second time to allow theincident pulse to pass through. According to embodiments of the presentinvention, the timing of the second “on” pulse of the optical amplitudemodulator 130 is synchronized with the first opening of the modulator130 (first “on” pulse) to take account of the transit time of the signalthrough the amplifier 150 and the reflecting structure 146. Afteremerging from the first side of the optical amplitude modulator 130, theamplified pulse then enters the second port 122 of optical circulator120, and exits from the third port 116 of optical circulator 120 alongoptical path 148.

According to embodiments of the present invention the drive signalapplied to the optical amplitude modulator 130 for the second openinghas a shaped waveform originating from a digital pattern converted intoan analog signal using a high speed Digital-to-Analog Converter (DAC) asdescribed for the first opening. This shaped waveform of the secondopening can be different than the waveform of the first openingdepending on the application at hand. In some embodiments, the seconddrive signal has merely a rectangular waveform to let the optical pulseexit the double-pass amplifier un-modified. In other embodiments, thesecond drive signal has a non-rectangular shape waveform to modify theoptical pulse on exit from the double-pass amplifier depending on theapplication at hand.

In yet other embodiments, the first drive signal and the second drivesignal are generated simultaneously as one complex waveform from thehigh-speed DAC, in a single trigger event. The process flow a user wouldfollow to generate this complex single electrical drive waveformincluding first and second opening signals is highlighted below. First auser would define an array of numbers representing the electricalvoltage applied to the modulator at every 1 nanosecond for the first andsecond openings. It will be apparent to someone in the art that thearray length will be at least as long as the total of twice the opticalpulse width and the transit time of the optical signal through thedouble-pass amplifier. For example, if the desired output optical pulseis 30 ns and the optical transit time through the amplifier is 150 ns,then the electrical waveform will be at least 210 ns. In someembodiments, the electrical waveform will be substantially zero betweenthe first opening and second opening signals. For a DAC of 1 GS/ssampling rate, the array length will preferably be more than 210numbers. Therefore, the DAC needs to store at least 210 samples. Morepreferably the DAC sample length can be longer than 1024. Then using acomputer, the array of numbers (the sample) is loaded into the DAC'smemory. Then every single trigger event would output the complex analogelectrical waveform represented by the array of numbers, synchronizedfor the first and second opening. The analog electrical waveform isapplied to the modulator. For example, an apparatus for generating suchwaveform is the model AWG2040 from Tektronix, Inc. of Beaverton, Oreg.

The signal is then amplified as it passes through a second opticalamplifier 160. As discussed in relation to FIG. 1, embodiments of thepresent invention utilize a fiber amplifier as optical amplifier 160,including a pump 154 that is coupled to a rare-earth-doped fiber loop156 through an optical coupler 152. Generally, a semiconductor pumplaser is used as pump 154, although pumping of optical amplifiers can beachieved by other means as will be evident to one of skill in the art.In a particular embodiment, the second optical amplifier 160 includes a5 meter length of rare-earth doped fiber 156, having a core diameter ofapproximately 4.8 μm, and is doped with Ytterbium to a doping density ofapproximately 6×10²⁴ ions/m³. The amplifier 160 also includes a pump154, which is an FBG-stabilized semiconductor laser diode operating at awavelength of 976 nm, and having an output power of 500 mW. In anotherparticular embodiment, the second optical amplifier 160 includes a 2meter length of rare-earth doped fiber 156, having a core diameter ofapproximately 10 μm, and is doped with Ytterbium to a doping density ofapproximately 1×10²⁶ ions/m³. The amplifier 160 can also includes a pump154, which is a semiconductor laser diode having an output power of 5 W.

In another particular embodiment, in order to pass the seed signal, theoptical amplitude modulator 130 is pulsed once instead of twice. Theoptical amplitude modulator 130 is turned to the “on” state to generatethe rising edge of the pulse propagating along optical path 136. Thissignal is then amplified a first time through optical amplifier 150. Thesignal then impinges on the reflecting structure 146 and is amplified asecond time through optical amplifier 150. Now the signal emerging fromoptical amplifier 150 along optical path 136 impinges on the second side134 of the optical amplitude modulator 130, which is subsequently turnedto the “off” state. The pulse width is therefore given by the timeduration during which the optical amplitude modulator 130 is held in the“on” state subtracted by the transit time of the signal through theamplifier 150 and the reflecting structure 146. The modulator drivesignal applied to the optical amplitude modulator 130 has a shapedwaveform originating from a digital pattern converted into an analogsignal using a high speed Digital-to-Analog Converter (DAC) as describedabove.

Although FIG. 1 illustrates the use of a single optical amplifier 160coupled to the third port of the optical circulator 120, this is notrequired by the present invention. In alternative embodiments, multipleoptical amplifiers are utilized downstream of the optical circulator 120as appropriate to the particular applications. One of ordinary skill inthe art would recognize many variations, modifications, andalternatives. Additional description related to optical sources utilizedin embodiments of the present invention can be found in commonlyassigned and co-pending U.S. patent application Ser. No. 11/737,052,filed on Apr. 18, 2007, and incorporated by reference herein in itsentirety for all purposes.

FIG. 2 is a simplified timing diagram illustrating electrical andoptical pulses at different locations in a high power pulsed laseraccording to an embodiment of the present invention. Merely by way ofexample, FIG. 2 illustrates the timing of repetitive electrical drivesignals to the amplitude modulator and optical pulses propagatingthrough an embodiment of the invention as described in FIG. 1. Followingan electrical trigger 210, a first electrical drive signal 220 isapplied to the amplitude modulator to generate an optical pulse 240.After some propagation delay, the optical signal 250 passes through theoptical amplifier a first time. The optical signal 260 then impinges onthe reflecting structure and passes through the optical amplifier asecond time 250. The optical pulses 240 are transmitted through theamplitude modulator a second time, which is driven electrically a secondtime 220 with the optical pulses 240. Finally the optical pulses 230exit port 3 of the circulator after some propagation delay.

Utilizing embodiments of the present invention, high power pulsed lasersources are provided that generate streams of optical pulses withindependently adjustable pulse characteristics including pulse width,peak power and energy, pulse shape, and pulse repetition rate. Merely byway of example, a particular embodiment of the present inventiondelivers output pulses at the output 170 of second optical amplifier 160of more than 5 μJ per pulse at a pulse width of 10 ns and at arepetition rate of 10 kHz. Of course, other pulse characteristics areprovided by alternative embodiments.

In the embodiments described above, a CW seed source is utilized andtime-domain filtering to provide a laser pulse is performed using theamplitude modulator 120. However, this is not required by the presentinvention. In an alternative embodiment, the seed signal is modulated toprovide a pulsed seed signal rather than CW seed signal. Providing apulsed seed signal minimizes ASE build-up and gain depletion caused byseed leakage through the amplitude modulator into the double-passamplifier and enables the operating power range of the seed source to beincreased. In this alternative embodiment, the pulsed seed signal may beof a pulse width equal to, or longer than the desired pulse width ofoverall pulsed laser source. Pulsing the seed can also increase theeffective linewidth of the seed laser to reduce Stimulated BrillouinScattering (SBS).

According to embodiments of the present invention, methods and systemsare provided that result in the generation of sequences of opticalpulses, which may not be equally separated in time. Moreover, the pulsewidths and pulse energies are individually tailored in a predeterminedmanner from pulse to pulse. Furthermore, it will be recognized thatalthough the above description discussed the generation of a singleoptical pulse, embodiments of the present invention provide for thegeneration of multiple pulses by repeating the single pulse amultiplicity of times. These multiple pulses may include a train ofoptical pulse sequences. In some embodiments of the present invention,the DAC generates the shaped waveform at every trigger event, thereforegenerating multiple optical pulses synchronized with every triggerevent. This mode of operation is advantageously used when optical pulsesare generated at rates up to 100 kHz, up to 500 kHz, or up to 1 MHz orhigher. In other embodiments, the DAC can be used to generate multipleoptical pulses with every trigger event. In this mode of operation theshaped waveform contains a set of pulses. In some embodiments, the setof optical pulses are identical. In other embodiments, the set ofoptical pulses are different. This mode of operation is particularlyadvantageous when bursts of optical pulses are required for a particularapplication. For example, it can be advantageous to generate two or more10 ns pulses with a 10-20 ns time delay, with the whole set of pulsesrepeated at a rate up to 10 kHz, up to 500 kHz, or up to 1 MHz orhigher. In this example, every single trigger event would generate theset of 10 ns pulses.

Laser-based material processing such as marking, engraving,micro-machining, and cutting has made extensive use of high peak powerpulse lasers. Depending on the applications and the material to beprocessed, the pulse characteristics need to be adapted for the task athand, and especially the shape. For several applications, it ispreferable to process with a specific optical pulse shape, such assquare pulse, and a deformation of such a pulse may not be desirable.

In fiber-based power amplifiers, shape deformation of the optical pulseoccurs when the output pulse energy approaches the stored energy withinthe amplifier. In the steady state regime it is the signal intensitythat is responsible for gain saturation effects, whereas for the dynamicregime it is the energy density. The main consequence is that a highpower pulse is distorted as it propagates through the gain medium. Asthe pulse passes through the fiber amplifier, it extracts progressivelymore energy from the fiber, reducing the available gain as it does so.It is this progressively reducing gain which causes the pulsedeformation. FIG. 3 is a pulse shape diagram for Gaussian andquasi-square (super-Gaussian) pulses. Because the Gaussian shapecontains less of its energy in the leading edge, the pulse deformationis qualitatively less severe. On the other hand, the effect of pulsedepletion on the quasi-square pulse shape is dramatic.

In the field of laser processing of conductive links on memory chips itcan be advantageous to use substantially square optical pulses.Preferably the rise and fall time can be around 1 nanosecond (ns). Mostpreferably the rise and fall time can be less than 1 ns, about 0.5 ns orless. In embodiments of the present invention, the gain saturation inthe amplifier is counter-balanced to generate a substantially squareoutput pulse by generating a driving signal for the modulator that has alower rising edge than falling edge. FIGS. 4A and 4B are simplifiedillustration of an electrical waveform applied to the amplitudemodulator and an output optical pulse according to an embodiment of thepresent invention. In this example, the desired output optical pulsepresented in FIG. 4B has an energy of 20 mJ, rise and fall time of 1 ns,a width of 30 ns, and a peak power of about 670 W. The electricalwaveform applied on the amplitude modulator to generate the desiredoutput optical pulse is shown in FIG. 4A. The electrical drive signal isgenerated using a DAC with 1 GS/s sampling rate and 12 bits resolution.In this simple illustration, only the electrical drive signal applied tothe modulator for the first time (first opening) is shown. FIG. 4Aillustrates the voltage applied to the amplitude modulator as a fractionof V_pi (V_(π)), which is well known in the art as the voltage requiredto drive the amplitude modulator from complete extinction to completetransmission.

In one embodiment, the modulator's electrical drive signal used thesecond time (second opening) is rectangular. This second opening onlylets the optical pulse exit the double-pass amplifier toward the outputamplifier unmodified. The second electrical drive signal merely gatesthe output optical pulse. In other embodiments, the second electricaldrive signal can also be shaped using a non-rectangular waveform,depending on the application. It can also be recognized, that the firstand second electrical drive signals can in fact be one long complexdrive signal containing the first and second drive waveforms asexplained above. The general feature of the electrical drive signal forthis example is that the leading edge is lower than the falling edge.This happens to take into account saturation in the amplifiers. As theleading edge of the pulse extracts energy from the amplifier at itpropagates through it, the optical gain is decreased. The falling edgeof the pulse seeing less gain is amplified less. Therefore the fallingedge of the pulse injected in the amplifier as to be higher than theleading edge.

In other embodiments of the present invention pulse shaping is used notonly to counteract gain saturation but to generate non-square opticalpulses. Depending on the application at hand, the output optical pulsecan be different than a square pulse.

In the field of laser processing of conductive links on memory chips orother integrated circuits chips, systems employing laser pulses havingspecially tailored power profiles can be advantageous for betterprocessing quality and yield.

For example in an embodiment of the present invention, a speciallytailored optical temporal power profile is used as illustrated in FIG.5A or 5B. With reference to FIGS. 5A and 5B, the laser pulse powerprofiles 60 c and 60 d, respectively, can be specially tailored to havea significant leading edge overshoot 62 (FIG. 5A) at the beginning ofthe laser pulse or have one or two mid-pulse spikes 64 (one spike shownin FIG. 5B) sometime within the duration of the laser pulse, before thelink material is totally removed. In a specific embodiment, the timingof the power spike is within an interval measured from the rising edgeof the laser pulse power profile to about 70% of the duration of thelaser pulse power profile. FIG. 5B shows laser pulse power profile 60 din which the power level is relatively flat before and after pulse spike64. The laser pulse power profile can have a changing power level beforeand after pulse spike 64. Tailoring the laser pulse power profile inthis manner provides from leading edge overshoot or mid-pulse spikessufficient laser peak power and energy to facilitate the satisfactoryremoval of the link material and, upon removal of most of the linkmaterial, much lower laser pulse power to remove remaining link materialand ensure reduced risk of damage to silicon substrate and to thestructure neighboring the link.

FIGS. 5C and 5D are simplified illustrations of an electrical waveformapplied to the amplitude modulator and the corresponding spiked opticaloutput pulse according to an embodiment of the present invention. Thesefigures illustrate a specific example for a mid-pulse spiked opticaloutput. In this example, the desired output optical pulse presented inFIG. 5D has an energy of 20 mJ, rise and fall time of about 5 ns, atotal width at the base of about 55 ns, a spike width of about 15 ns atthe base, and a peak power of about 570 W. The output pulse has a spikein the center of the main pulse. The pulse has a pedestal at about 285W, at about half power from the peak power of 570 W. The electricalwaveform applied on the amplitude modulator to generate the desiredoutput optical pulse is shown in FIG. 5C. The electrical drive signal isgenerated using a DAC with 1 GS/s sampling rate and 12 bits resolution.In this simple illustration, only the electrical drive signal applied tothe modulator for the first time (first opening) is shown. FIG. 5Cillustrates the voltage applied to the amplitude modulator as a fractionof V_pi (V_(π)), which is well known in the art as the voltage requiredto drive the amplitude modulator from complete extinction to completetransmission. In one embodiment, the modulator's electrical drive signalused the second time (second opening) is rectangular. This secondopening only lets the optical pulse exit the double-pass amplifiertoward the output amplifier unmodified. The second electrical drivesignal merely gates the output optical pulse. In other embodiments, thesecond electrical drive signal can also be shaped using anon-rectangular waveform, depending on the application. It can also berecognized, that the first and second electrical drive signals can infact be one long complex drive signal containing the first and seconddrive waveforms as explained above.

Utilizing embodiments of the present invention, such special tailoringof the laser power profile delivers much better processing results and awider process window and reduces risk of damage to the silicon substrateand to the structure adjacent to the link.

The specially tailored laser pulses provided herein have either anovershoot at the beginning of the laser pulse or a spike peak within theduration of the laser pulse. The power amplitude of the overshoot or thespike peak during the pulse is more than about 10%, for example, 10% to50%, over the average power amplitude of the laser pulse. The temporalwidth of the overshoot or the spike peak is a predetermined value, forexample, between about 1 ns and about 50% of the duration of the laserpulse. In a particular embodiment, the temporal width of the overshootor the spike peak is between about 10% and about 50% of the duration ofthe laser pulse. In an embodiment, the timing of the spike is set aheadof the time when the link is totally removed, considering all therealistic variations of the link structure and laser parameters duringmanufacturing. Other techniques of modulating the laser pulse temporalpower profile can be used, such as multiple leading edge overshoots,multiple spike peaks, or oscillating peak power amplitude, based ondifferent link structures. In some applications, the duration of thelaser pulse is between about 1 ns and about 40 ns. The falling edge ofthe laser pulse temporal power profile is typically shorter than about10 ns. The energy of the laser pulse is preferably between about 0.001microjoule and about 10 microjoule.

Another example of non-square pulses provided according to a specificembodiment of the present invention are the sets of laser pulsesillustrated in FIG. 6. Yields in integrated circuit (IC) fabricationprocesses often incur defects resulting from alignment variations ofsubsurface layers or patterns or particulate contaminants. For suchapplications, it is advantageous to employ sets of at least two laserpulses, each with a laser pulse energy within a safe range, to sever anIC link, instead of using a single laser pulse of conventional linkprocessing systems. In some embodiments of the present invention, setsof optical pulses similar to the ones illustrated in FIG. 6 are emittedby a laser. According to various embodiments of the present invention,the duration of the set of optical pulses is shorter than 1,000 ns,shorter than 500 ns, shorter than 300 ns, or in the range of 5 ns to 300ns. Of course, the particular values for the pulse width will depend onthe particular applications. The pulse width of each laser pulse withinthe set is generally in the range of about 100 femtoseconds to about 30ns. In an embodiment used for IC link severing applications, each laserpulse within the set has an energy or peak power per pulse that is lessthan the damage threshold for the silicon substrate supporting the linkstructure. The number of laser pulses in the set is provided as apredetermined number such that the last pulse cleans off the bottom ofthe link while leaving the underlying passivation layer and thesubstrate intact. Because the whole duration of the set is typicallyshorter than 1,000 ns, the set is considered to be a single pulse by atraditional link-severing laser positioning system. According to anembodiment of the present invention, a set of laser pulses is created bygenerating a digital representation of the proper modulator electricalwaveform in a computer and then converting this digital pattern into ananalog signal using a high speed Digital-to-Analog (DAC) converter. Insome embodiments, the set of pulses is treated as one single opticalwaveform, which only uses one single electrical signal to drive theamplitude modulator.

In some embodiments the set of pulses comprises pulses of differentshapes, energies, or widths. For example, a simple set comprises threepulses of different energies or widths as shown in FIGS. 7A and 7B.FIGS. 7A and 7B are simplified illustrations of an electrical waveformapplied to the amplitude modulator and an output optical pulse accordingto an embodiment of the present invention for three pulses of differentenergies and widths. In this example, the desired output opticalwaveform presented in FIG. 7B has a total energy of 50 mJ. This firstpulse has an energy of 6 mJ, rise and fall times of about 1 ns, a widthof 6 ns, and a peak power of about 1000 W. The second pulse has anenergy of 4 mJ, rise and fall times of about 1 ns, a width of 8 ns, anda peak power of about 500 W. The third pulse has an energy of 40 mJ,rise and fall times of about 1 ns, a width of 50 ns, and a peak power ofabout 800 W. The first two pulses are separated by 20 ns, and the secondand third pulses are separated by 50 ns. The pulse separation and thefact that the three pulses have different widths and powers is generallyvery difficult to achieve with traditional methods. In this embodimentthe three pulses are treated as a single waveform. The electricalwaveform applied on the amplitude modulator to generate the desiredoutput optical pulse is shown in FIG. 7A. The electrical drive signal isgenerated using a DAC with 1 GS/s sampling rate and 12 bits resolution.In this simple illustration, only the electrical drive signal applied tothe modulator for the first time (first opening) is shown. FIG. 7Billustrates the voltage applied to the amplitude modulator as a fractionof V_pi (V_(π)). In one embodiment, the modulator's electrical drivesignal used the second time (second opening) is rectangular.

In yet another embodiment of the present invention, an optical waveformfor efficient laser processing is generated. The shape of the laserpulse is described next. First laser energy is applied to a materialsurface for a predetermined period t₁ at a predetermined power level P₁such that processed material melts and evaporates producing evaporationinduced recoil which displaces produced melt to a periphery of theinteraction zone without substantial ejection of molten material fromthe laser beam interaction zone, thereby creating a keyhole in thematerial. Then, more laser energy is applied to the laser beaminteraction zone for a predetermined period t₂ at a differentpredetermined power level P₂ such that the induced evaporation recoil isinsufficient to counteract surface tension pressure, and a controlledcollapse of said keyhole occurs while maintaining a temperature ofproduced melt above a melting temperature of the material. Third, yetmore laser energy is applied to the laser beam interaction zone for apredetermined period t₃ at a different predetermined power level P₃,such that rapid evaporation of the molten material is induced,generating recoil which produces near complete ejection of the meltcreated during the first period to, and thus forms a crater. Finally,laser energy is applied to the laser beam interaction zone for apredetermined period t₄ at a different predetermined power level P₄,such that a temperature of the remaining melt which was not ejected fromthe interaction zone and a temperature of the solid material near a wallof said crater decrease at a controlled rate which is below a coolingrate which would ordinarily produced microcracking of the material.

FIGS. 8A and 8B are simplified illustrations of an electrical waveformapplied to the amplitude modulator and an output optical pulse accordingto yet another embodiment of the present invention. An application inwhich the embodiment illustrated in FIG. 8 may be used is the efficientlaser processing of materials as described above. In this example, thedesired output optical waveform presented in FIG. 8B has a total energyof 20 mJ. First laser energy is generated during a period of about 20 nsat a power level of 175 W. Then laser energy is generated during aperiod of about 25 ns at a power level of 88 W. Yet more laser energy isgenerated during about 10 ns at a power level of 690 W. Finally morelaser energy is generated during a period of 55 ns at a power level of88 W. The electrical waveform applied on the amplitude modulator togenerate the desired output optical pulse is shown in FIG. 8A. Theelectrical drive signal is generated using a DAC with 1 GS/s samplingrate and 12 bits resolution. In this simple illustration, only theelectrical drive signal applied to the modulator for the first time(first opening) is shown. FIG. 8A illustrates the voltage applied to theamplitude modulator as a fraction of V_pi (V_(π)). In one embodiment,the modulator's electrical drive signal used the second time (secondopening) is rectangular.

In some applications, the output of the laser system is frequencyconverted to generate signals at other wavelengths. For example, signalsat 1064 nm can be frequency doubled or tripled to generate 532 nm, or354 nm signals, as is well known in the art. Units used for frequencyconversion usually contains one or more nonlinear optical crystals suchas LBO, KTP, or the like. The optical time domain waveform of thefrequency converted signal has a nonlinear relationship with the opticaltime domain waveform of the signal at the fundamental wavelength. Forexample, for frequency doubling, the two waveforms are approximatelyrelated by a power of two relationships. This means that if the power ofthe fundamental frequency is doubled, the power at the double frequencyis quadruple. For example if the peak power at 1064 nm is increased from10 kW to 20 kW, the peak power at 532 nm will approximately bemultiplied by a factor of four, if all other parameters are kept asconstants. The 532 nm peak power might go from 2.5 kW to 10 kW underthese conditions. The frequency conversion of a complex optical waveformis then not linear. Therefore if a certain frequency converted opticalwaveform is required for a given application, the fundamental pulseshape will have a different waveform. This difference needs to be takeninto account in the electrical drive signal applied to the amplitudemodulator. According to some embodiments of the present invention, theelectrical drive signal applied to the amplitude modulator take intoaccount optical waveform distortion due to the frequency conversionprocess.

As discussed above, the particular pulse shaped waveform applied to theamplitude modulator depends on the application at hand. It also dependson the particular amplifier architecture being used. A lower poweramplifier using single mode core fibers will not necessarily require thesame amount of pulse shaping than a higher power amplifier using largemode fibers. Also if chains of amplifiers are used the particular shapedwaveform will depend on the gains and the fiber geometries used in eachstage. It also depends on the presence or not of frequency conversionunits. Therefore for a particular output optical waveform, one has todetermine the optimal electrical waveform applied to the amplitudemodulator. To determine the optimal electrical waveform, one can useseveral methods. A first one is to develop a physical model of the lasersystem that relates the modulator's electrical drive waveform to theoptical output waveform. By doing numerical simulations for a particularsystem, it is then possible to determine the optimal electrical drivewaveform. The inventors of the present application have developed such amodel. Another approach consists in using successive approximations on alaser system in the laboratory to determine the best electricalwaveform. In this approach, a first electrical waveform is applied tothe amplitude modulator, which generates a first optical output waveformfrom the laser system. Then the optical output and electrical waveformsare compared to generate an error signal that modifies the firstelectrical waveform. An error signal can be calculated for example usinga computer that controls a fast DAC generating the electrical waveformas discussed in embodiments of the current invention. One such errorsignal can be generated by subtracting the normalized inverse of thefirst optical output waveform from the first electrical waveform. Thiserror signal is then added to the first electrical waveform to generatea second electrical waveform driving the modulator to generate a secondoptical output waveform. The sequence is then repeated until theelectrical waveform driving the amplitude modulator generates therequired optical output waveform.

According to one particular embodiment of the present invention, FIG. 9shows an exemplary laser processing system 900. The system 900 includesa laser source 902, a wavelength converter 906, a DAC 914, an opticalsystem 910, a controller 918, a sensor 922, and a workpiece 926 that ispositioned on top of a workpiece holder 930. The laser source 902provides laser pulses with certain characteristics, such as wavelength,pulse length, pulse shape, and pulse repetition rate. The wavelength maybe selected by the controller 918. The wavelength may also be adjustedthrough the wavelength converter by using the controller 918. The pulselength, pulse shape, and pulse repetition rate may be adjusted by thecontroller 918 through the DAC 914 according to the embodiment of thepresent invention. The controller 918 may provide information forprocessing a particular material, such as optimal pulse shape and pulselength for processing the particular material.

A wavelength generated by the laser source 902 may be converted to aharmonic of a fundamental wavelength by the wavelength converter 906,such as a second, third, or fourth harmonic wavelength. Although somesystems use different lasers, it is possible to obtain differentwavelengths from one laser using a well-known process of harmonicgeneration in non-linear crystals. For example, ultraviolet light havinga wavelength of approximately 353 nm may be obtained from an infraredlaser having a wavelength of 1.06 μm by using harmonic tripling in anon-linear crystal. The wavelength converter 906 may include a beamdirecting device, such as galvanometer-mounted mirrors. The mirrors mayquickly change the path of a laser beam from the laser source 902 tobypass the wavelength converter 906 by using the controller 918.

The optical system 910 may be used to adjust beam shape or spot size ofthe beam. The optical system 910 may include lenses and mirrors forfocusing a laser beam on the workpiece 926, and a component fordirecting the beam to various positions on the workpiece 926. In aspecific embodiment, the component for directing the beam may be mirrorsmounted on galvanometers. The controller 918 may be used to control theoptical system 910 and the motion of the component for directing beam.For example, when cutting a hole in the workpiece 926 in a trepanningprocess, the optical system 910 may be controlled by the controller 918to scan the beam in a circle on the area where the hole needs to be cut.Alternatively, when cutting a hole in the workpiece 926 in a percussionprocess, a laser beam is directed toward an area where the hole needs tobe cut and may be pulsed multiple times to drill the hole directly. Thelaser beam may process each small area of the workpiece 926 held on aworkpiece holder 930 with a movable stage by moving the workpiece holder930 controlled by the controller 918.

Many applications may require multiple processing steps by usingdifferent laser beams. In some cases, the process may not bereproducible enough so as to allow precise prediction of a particulartime to change laser parameters. In such cases, the laser system 900 mayuse the sensor 922 as an indicator to detect and indicate that one ofthe process steps has been completed. The sensor 922 may then provide afeedback signal to the controller 918 that is in communication with thelaser source 902 through the DAC 914 to switch to another processingstep. One benefit of using the sensor 926 is that the informationobtained by the sensor 926 may be used to provide a feedback signal tothe controller 918 to change or optimize laser parameters with no delayin laser processing.

There are many ways of using sensors to monitor a processing sequence orstep. In one embodiment of the invention, the sensor 922 may be a visionsystem for viewing the workpiece 926 as the laser processing occurs,such as a video camera. In another embodiment of the invention, thesensor 922 may be a photodiode for detecting an indicator such as achange in the light emitted from the workpiece 926. In yet a furtherembodiment of the invention, the sensor 922 may be an audio detectornear the workpiece 926 for detecting an indicator such as a change inpitch or loudness of sound during a laser processing. One of ordinaryskill in the art would recognize many variations, modifications, andalternatives.

Lasers may be used in processing a workpiece 926 that comprises ahomogeneous material or a multilayer structure of different materials.For example, a laser is often used to remove a metal conductor betweentwo contacts where the metal forms part of a circuit deposited on aninsulating substrate that comprises a glass or a dielectric material.The workpiece 926 may be a multilayer structure having at least twolayers of different materials.

FIG. 10A shows an example of a thin layer metal 1002 on a glasssubstrate 1006. The metal may be aluminum. FIG. 10B shows that a portionof the metal layer 1002 is removed by using a first set of laser pulses.Three regions are formed, including a region 1018 that contains somedebris 1014 generated from the removal of the metal layer 1002, regions1010 a and 1010 b that are the remaining portions of the metal layer1002 on each side of the region 1018. The first set of laser pulsesincludes two high energy and short pulses for quick removal of a portionof the metal layer 1002.

Referring to FIG. 10C now, the debris 1014 in the region 1018 is removedor cleaned by using a second set of laser pulses. A first set of laserparameters is changed to a second set of laser parameters when the metallayer 1002 is removed from the glass substrate 1006. The second set oflaser pulses includes five low energy and long pulses for cleaning thedebris 1014 without damaging the glass substrate 1006.

In a specific embodiment of the invention, the laser wavelength may be1.06 μm and a ytterbium-doped fiber amplifier may be used. In the firstprocess of partially removing the metal layer 1002, the short pulses orthe first set of laser pulses may have a pulse shape as shown in FIG.3B, a pulse length of 5 ns and a pulse energy of 0.2 mJ. In the secondprocess of cleaning the debris 1014, the long pulses or the second setof laser pulses may have a pulse length of 100 ns and a pulse energy of0.05 mJ. In both the first and second processes, a pulse repetition rateof 20 kHz may be used.

Another common application may use lasers to drill via holes through amultilayer circuit board, where each of the multilayer alternatesbetween a conductor (e.g. copper) and an insulator (e.g. glass filledepoxy or thermoplastic). Each layer may require a different set of laserparameters to optimize laser processing for the particular material ofthe circuit board. For example, long square pulses may be used forannealing or cleaning, while short pulses or ultrafast pulses may beused for precise removable of small amounts of materials. Although theshort pulses may produce a very clean or sharp edged removal ofmaterials, the removable rate from the short pulses may be lower thanthe long pulses because less energy is associated with the short pulses.Therefore, it is important to select pulse shape, pulse length, pulseenergy for a particular material to optimize the laser processing.

FIG. 11 shows a simplified sectional diagram of a circuit board. Thecircuit board 1100 comprises a first layer 1106 of a first material, asecond layer 1110 of a second material, and a third layer 1114 of thefirst material, a fourth layer 1118 of the second material and asubstrate 1122. In some embodiments, the substrate or the circuit boardare referred to as a workpiece. Round via hole 1102 a is drilled throughthe first and second layers 1106 and 1110 by using laser pluses, andround via hole 1102 b is drilled through the third and fourth layers1114 and 1118. The first material may be a metal, such as copper, whilethe second material may be a polymer, such as polyimide. The first andsecond layers 1106 and 1110 may have a different size of via 1102 a thanthe via 1102 b in the third and fourth layers 1114 and 1118.

In a specific embodiment of the invention, the laser source 902 may havea fundamental wavelength of 1.06 μm. A ytterbium-doped fiber amplifier144 or 156 may be used with the laser source 902. When drilling throughthe first layer 1106 and the third layer 1114 of the first material(e.g. copper), a third harmonic wavelength of the laser source 902 (353nm ultraviolet) may be used, because the fundamental wavelength of 1.06μm is substantially reflected by metals like copper. Laser parametersmay include a pulse energy of 0.05 mJ, a pulse length of 5 ns, and thepulse shape as shown in FIG. 3B. The ultraviolet light may be focused ona small spot size to provide a high energy density of at least 50 J/cm².Vias 1102 a and 1102 b may be formed by a trepanning drilling in thefirst and third layers 1106 and 1114 of the first material (e.g. copper)with laser pulses at a pulse repetition rate of 50 kHz.

When drilling through the second layer 1110 and fourth layer 1118 of thesecond material (e.g. polyimide), a fundamental wavelength of 1.06 μmmay be used by configuring mirrors to direct the laser beam from thelaser source 902 to bypass the wavelength converter 906. Via holes 1102a and 1102 b in the second and fourth layers 1110 and 1118 may be formedby a percussion drilling with a pulse energy of 0.5 mJ, a pulse lengthof 100 ns and a pulse shape similar to that shown in FIG. 4B, and apulse repetition rate of 10 kHz. A total number of pulses may be morethan 100 pulses.

In a particular embodiment, an assistant gas flow may be used to helpclean the debris generated in laser processing. A vision system may beused as a sensor 922 in determining when to switch the laser parameters.The vision system may detect the brightness and spectral information offlames to indicate whether the material being processed is a metal or aplastic.

One benefit of using different sets of laser parameters according to theembodiment of the invention is to process an entire workpiece with morethan one pass, but without any time delay between two consecutivepasses, as the time required for adjusting laser parameters is shorterthan the time between consecutive laser pulses. This technique mayrequire substantially less processing time than the case where at leasttwo different lasers are required in processing a multilayer structure.When using at least two different lasers, it is difficult andtime-consuming to re-achieve the alignment on each of the areas forsuccessive laser drilling or cleaning, where each such area has beenprocessed in a previous pass or previous passes.

In another embodiment of the present invention, processing can takeplace in a single pass over the workpiece rather than in multiplepasses, which can occur if more than one laser is used. In single passembodiment, a benefit is that throughput is maximized in comparison withconventional processing since the time taken to move the workpiece beingprocessed (e.g., a printed circuit board) to various positions isusually greater than the time taken to process the workpiece (e.g.,drilling a set of vias). Hence, having a single pass can reduce thetotal processing time. Thus, this embodiment can provide significantbenefits in throughput by using the flexible laser systems describedthroughout the present specification.

Furthermore, the technique of using different sets of laser parametersis also better than using a single set of parameters to processdifferent materials. When a single set of parameters is used inprocessing a workpiece having different materials, the parameters maynot be optimized for any of the materials so that laser processing maytake longer time or may result in undesirable side effects, such aspitting, ridging, or burn zone etc.

While the present invention has been described with respect toparticular embodiments and specific examples thereof, it should beunderstood that other embodiments may fall within the spirit and scopeof the invention. The scope of the invention should, therefore, bedetermined with reference to the appended claims along with their fullscope of equivalents.

1. A laser system for processing a workpiece, the laser systemcomprising: a tunable pulsed laser source having an output comprising aset of optical pulses, wherein the tunable pulsed laser sourcecomprises: a seed source adapted to generate a seed signal; an opticalcirculator having a first port coupled to the seed source, a secondport, and a third port; a modulator driver adapted to produce anelectrical waveform; an amplitude modulator coupled to the modulatordriver and adapted to receive the electrical waveform, wherein theamplitude modulator is characterized by a first side coupled to thesecond port of the optical circulator and a second side; a first opticalamplifier characterized by an input end and a reflective end, whereinthe input end is coupled to the second side of the amplitude modulator;and a second optical amplifier coupled to the third port of the opticalcirculator; a controller configured to adjust laser parameters of thetunable pulsed laser source; a supporting member configured to supportthe workpiece; and an optical system configured to adjust laser beamsfrom the tunable pulsed laser source and direct them towards theworkpiece.
 2. The laser system of claim 1 further comprising awavelength converter.
 3. The laser system of claim 2 wherein thewavelength converter comprises a harmonic generator configured togenerate harmonic wavelengths from a fundamental wavelength.
 4. Thelaser system of claim 3 wherein the wavelength converter furthercomprises a plurality of mirrors configured for a laser beam from thetunable pulsed laser source to bypass the wavelength converter.
 5. Thelaser system of claim 3 wherein the harmonic generator comprises atleast one non-linear crystal.
 6. The laser system of claim 1 furthercomprising a Digital-to-Analog Converter (DAC) configured to provideelectrical waveforms.
 7. The laser system of claim 1 wherein theworkpiece comprises at least a first layer and a second layer, the firstlayer and the second layer comprising different materials.
 8. The lasersystem of claim 7 wherein the set of optical pulses comprises: a firstset of pulses characterized by a first set of shapes, a first set ofdurations and a first set of intensities; and a second set of pulsescharacterized by a second set of shapes, a second set of durations, anda second set of intensities, wherein the first set of pulses isdifferent from the second set of pulses.
 9. The laser system of claim 8wherein the first set of pulses is used in processing the first layerand the second set of pulses is used in processing the second layer. 10.The laser system of claim 1 further comprising a sensor coupled to thecontroller and configured to detect a phase of laser processing.
 11. Thelaser system of claim 10 wherein the phase of laser processing comprisesan end of laser processing.
 12. The laser system of claim 1 furthercomprising a digital-to-analog converter configured to generate theelectrical waveform, the digital-to-analog converter being characterizedby a sampling rate of 500 megasamples/second or faster and an analogelectrical bandwidth larger than 100 MHz.
 13. A method of laserprocessing, the method comprising: outputting a first set of opticalpulses from a tunable pulsed laser source toward a workpiece, whereinthe workpiece comprises at least a first layer of a first material and asecond layer of a second material, the first material being differentfrom the second material, wherein the tunable pulsed laser sourcecomprises: a seed source adapted to generate a seed signal; an opticalcirculator having a first port coupled to the seed source, a secondport, and a third port; a modulator driver adapted to produce anelectrical waveform; an amplitude modulator coupled to the modulatordriver and adapted to receive the electrical waveform, wherein theamplitude modulator is characterized by a first side coupled to thesecond port of the optical circulator and a second side; a first opticalamplifier characterized by an input end and a reflective end, whereinthe input end is coupled to the second side of the amplitude modulator;and a second optical amplifier coupled to the third port of the opticalcirculator; processing the first layer using the first set of opticalpulses; outputting a second set of optical pulses from the tunablepulsed laser source toward the workpiece, wherein the second set ofoptical pulses is different from the first set of optical pulses; andprocessing the second layer using the second set of optical pulses. 14.The method of claim 13 wherein at least one of the first set of opticalpulses or the second set of optical pulse are characterized by afundamental wavelength, the method further comprising converting thefundamental wavelength to a harmonic wavelength.
 15. The method of claim13 further comprising: detecting a signal associated with a terminationof processing of the first layer; and receiving the detected signal at acontroller coupled to the tunable pulsed laser source.
 16. The method ofclaim 13 wherein: the first set of optical pulses comprises a first setof pulses characterized by a first set of shapes, a first set ofdurations, and a first set of intensities; and the second set of opticalpulses comprises a second set of pulses characterized by a second set ofshapes, a second set of durations, and a second set of intensities. 17.The method of claim 13 wherein a duration for switching from the firstset of optical pulses to the second set of optical pulses is shorterthan a duration between any two consecutive optical pulses.
 18. Themethod of claim 13 wherein laser processing comprises at least one ofdrilling, cutting, or cleaning.